[Patent document 1] JP-2004-260477 A
[Patent document 2] JP-2000-197093 A
[Patent document 3] JP-2005-20458 A (US 2004/0264412 A1)
[Patent document 4] JP-2006-310973 A
[Patent document 5] JP-2008-98880 A (US 2009/0270105 A1)
There is conventionally known a wireless communications system to provide wireless communications with a movable terminal by arranging several base stations so as to provide wireless communications areas to be overlapped with each other. Such a system uses a handover process (also referred to as an HO process) to switch automatically base stations to be connected with the terminal in response to the movement of the terminal. The HO process is required to operate so as not to reduce a communications quality.
As starting, the HO process searches another base station that provides a more effective electric wave state or level for communications. Next, the HO process switches a connection destination from the base station presently connected or communicating into the base station providing the more effective electric wave state.
Thus, the resources for data communications are restrained for the duration corresponding to the search and the switchover. As a result, there may be arising a lack of data such as a packet loss. Furthermore, before completing the HO process, the terminal may exit from the wireless communications area of the base station presently connected; thus, there may be arising an interruption of data communications. For example, the terminal may move at a high speed when the HO process is executed to switch from one base station to another base station. Such a case may cause a communications quality to deteriorate temporarily or the data communications to interrupt. The terminal executes an application, which needs successive data communications, like streaming of moving images or sounds. Under such an operating environment, there is arising a problem such as a partial lack of images or sounds, or a halt for re-buffering. Thus, there is a problem that a high communications quality is not provided to a user of the terminal.
In order to solve such a problem, there are conventionally proposed apparatuses and methods for the HO process, as described in Patent documents 1 to 5.
The technology of Patent documents 1, 2 sets up a threshold value for starting a HO process according to the moving speed of the terminal. In such a technology, as the moving speed becomes higher, the threshold value becomes higher. As the moving speed becomes higher, the HO process is started at an earlier stage.
It is noted that, in such a technology, the threshold value relative to a certain moving speed is not changed. Therefore, the communications quality may deteriorate during the HO process depending on an electric wave variation factor which fluctuates the electric wave state between the base station and the terminal. For example, the electric wave variation factor includes a difference in the reception sensitivity between terminals, a difference in the reception sensitivity between the models of terminals, and a difference in the communications environment due to the positional relation between the base station and the terminal.
The technology of Patent document 3 discloses a technique to select a suitable base station from several base stations. However, this technology cannot prevent the deterioration of the electric wave state in case that the terminal moves at a high speed. In addition, the communications quality may deteriorate during the HO process depending on the electric wave variation factor.
The technology of Patent document 4 uses a value, which is so-called a quality offset, so as to determine whether to start the HO process. Further, the quality offset is decreased when the HO process is successful, to thereby start a succeeding HO process at an earlier stage. Further, the quality offset is increased when the HO process is unsuccessful, to thereby start the next HO process at a later stage. This signifies that the above technology adjusts the starting condition of the HO process depending on whether the precedent HO process is successful or unsuccessful. However, in this conventional technology, the start condition of the HO process is not improved until the HO process once fails. Thus, there is a problem that a high communications quality may not provided to a user of the terminal.
The technology of Patent document 5 discloses implementation of two threshold values: the first threshold value serving as a start condition in case a HO permission instruction is issued from an application; and the second threshold value serving as a start condition in case a HO prohibition instruction is issued from an application. It is noted that this technology provides both of the threshold values to be fixed values, respectively. This cannot prevent the deterioration of the electric wave state in case that the terminal moves at a high speed. In addition, the communications quality may deteriorate during the HO process depending on the electric wave variation factor.
Thus, in the conventional technologies, the threshold value for starting the HO process may not be set up appropriately. Therefore, there is a problem that deterioration of the communications quality is not suppressed in the HO process.